1. Field of the Invention
The present invention relates generally to memory devices used in computerized control systems and more particularly to an improved static random access memory device (SRAM) having on-chip latches and associated control logic operative to save space and power and reduce component count and manufacturing cost in typical applications.
2. Description of the Prior Art
The decade of the eighties marked the beginning of rapid growth of microprocessors and microcontrollers for real-time embedded control applications. This trend is expected to continue during the 1990's as new integrated circuit components with higher performance and functionality will drive new applications. Advances in microcontroller architecture and process technology have precipitated highly integrated single-chip controllers with larger on-chip storage capacity and more I/O functions. In the midst of this explosive technological change one fact remains constant: the requirements for off-chip memory as program storage and data buffering continues to play a significant role. In most cases, traditional off-chip storage has been mostly used for control programs such as, for example, a set of control instructions for accomplishing a particular task. As such, EPROM (or OTP or ROM) devices are convenient and cost-effective devices to utilize. However, as new applications demand more intelligence and flexibility, read-modify-write operations are becoming an integral part of the entire application. Low-cost static random access memory (SRAM) is an optimum choice to complement and supplement the EPROM or OTP in new control environments. SRAM offers much more flexibility in that it allows stored information to be changed on-line, be it data or program instructions. Furthermore, as the clock frequency of microcontrollers forges toward 15 MHz to 25 MHz, SRAM appears to be the only viable and economical alternative for these cost-sensitive applications.
High-volume embedded control applications, such as intelligent hard-disk, tape and printer applications, continually require reductions in space and cost. By definition, the controller in an embedded control application is physically integrated or embedded inside the mechanism which it is controlling. More often than not, the circuit board space is at a premium. This condition puts great pressure on the system's designers who in turn must look to the semiconductor manufacturers for assistance with solutions that meet the stringent need of absolute minimum chip count, surface-mount packaging, and extremely low power consumption.